CHRONOLOGY OF THE HIGH-PRESSURE METAMORPHISM OF NORWEGIAN GARNET PERIDOTITES PYROXENITES

Eclogite facies mineral assemblages are variably preserved in mafic and ultramafic rocks within the Western Gneiss Region (WGR) of Norway. Mineralogical and microstructural data indicate that some Mg-Cr-rich, Alpine-type peridotites have had a complex metamorphic history. The metamorphic evolution of these rocks has been described in terms of a seven-stage evolutionary model; each stage is characterized by a specific mineral assemblage. Stages II and III both comprise garnet-bearing mineral assemblages. Garnet-bearing assemblages are also present in Fe-Ti-rich peridotites which commonly occur as layers in mafic complexes. Sm-Nd isotopic results are reported for mineral and whole rock samples from both of these types of peridotites and related rocks. The partitioning of Sm and Nd between coexisting garnet and clinopyroxene is used to assess chemical equilibrium. One sample of Mg-Cr-type peridotite shows non-disturbed partitioning of Sm and Nd between Stage II garnet and clinopyroxene pairs and yields a garnet-clinopyroxene-whole-rock date of 1703 +/- 29 Ma (I = 0.51069, MSWD = 0.04). This is the best estimate for the age of the Stage II high-P assemblage. Other Stage II garnet-clinopyroxene pairs reflect later disturbance of the Sm-Nd system and yield dates in the range 1303 to 1040 Ma. These dates may not have any geological significance. Stage III garnet-clinopyroxene pairs typically have equilibrated Sm-Nd partitioning and two samples yield dates of 437 +/- 58 and 511 +/- 18 Ma. This suggests that equilibration of the Stage III high-P assemblage is related to the Caledonian orogeny and is more or less contemporaneous with high-P metamorphism of 'country-rock' eclogites in the surrounding gneisses. The Sm-Nd mineral data for the Fe-Ti-rich garnet peridotites, are consistent with a Palaeozoic high-P metamorphism. Finally a synoptic P-T-t path is proposed for the Mg-Cr-type peridotites which is consistent with the petrological and geochronological data.